1. Field of the Invention
The invention concerns a process for loading fibers contained in a pulp suspension with calcium carbonate.
2. Description of the Related Art
Pulp suspensions of the above-mentioned type are used especially in paper and cardboard manufacture. The sparing use of raw material resources, due especially to economical and ecological concerns, is reflected in the paper production industry by the use of paper web with lower basis weights, as well as by the partial replacement of pulp with filling materials. If lower cost raw materials are used, the paper quality should at least be maintained. Among other things, the end product""s strength, visual characteristics, and processability play key roles in this challenge.
The present invention relates to a process and apparatus for loading fibers with calcium carbonate, especially in terms of optimum reaction balance, optimum reaction speed and optimum flexibility of production output.
The invention comprises, in one form thereof, a process of fiber loading that includes adding calcium oxide and/or a medium containing calcium hydroxide to a pulp suspension, and then charging the pulp suspension in several reactors with pure carbon dioxide or a medium containing carbon dioxide. Upon the addition of calcium oxide and/or a medium containing calcium hydroxide to the pulp, an exothermic chemical reaction takes place, the calcium hydroxide being added in liquid form (lime milk). By using lime milk, the water settled in or on the pulp suspension""s fibrous material is not necessarily required to start and continue the chemical reaction.
Because of this development, the chemical process, which is the basis of the loading, is split into several small processes, thereby permitting an optimum reaction balance, an optimum reaction speed, and an optimum flexibility of production output to be achieved. This use of several small processes enables a targeted and optimal testing of partial reactions, switching on and off of partial reactors and a variation of the operating parameters in the partial reactors.
On loading the fibers, calcium carbonate is imbedded into the wetted fiber surfaces by adding calcium oxide and/or calcium hydroxide to the wet fibrous material. Thereby, at least part of this calcium compound can associate with the water of the pulp mass. The so treated fibrous material is then charged with the pure carbon dioxide or the medium containing carbon dioxide.
As a result, the term xe2x80x9cwetted fiber surfacexe2x80x9d can include all wetted surfaces of the individual fibers. Consequently, the fibers become loaded with calcium carbonate both on their outer surfaces and on their insides (lumen).
Accordingly, the fibers are loaded with the filler material calcium carbonate, whereby the accumulation on the wetted surfaces takes place by a so-called xe2x80x9cFiber Loading(trademark)xe2x80x9d process, as described in U.S. Pat. No. 5,223,090. In this xe2x80x9cFiber Loading(trademark)xe2x80x9d process, the carbon dioxide reacts with the calcium hydroxide to form water and calcium carbonate.
For one suitable arrangement of the process in conformity with the invention, at least one section of the reactor is connected in series. Several smaller reaction volumes are created from one large reaction volume, so that the reaction speed is increased and, correspondingly, the contact surfaces of the reactants are enlarged. Furthermore, in an advantageous manner, (crystallization-) processes can be achieved in a targeted fashion and the parameters for an optimal reaction can be matched.
With a further advantageous arrangement, at least one section of the reactors is connected in parallel. In addition the above-mentioned advantages, this arrangement also allows for optimum adaptability of the production output. It is also of advantage, in parallel-connected reactors, to produce calcium carbonate with different crystal types and, preferably, to mix these on termination of the production process. This mixing allows an optimal composition of the product.
In certain cases, a combination of series- and parallel-connection of the reactors are of benefit. In this way, the parallel section can be matched to the required production range.
In other respects, the loading of fibers with calcium carbonate is as described in U.S. Pat. No. 5,223,090. The content of that patent is hereby incorporated by reference herein.
The apparatus embodying the invention essentially includes several reactors, in which the pulp suspension, mixed with calcium oxide and/or calcium hydroxide, can be charged with pure carbon dioxide or a medium containing carbon dioxide.